This namespace take some classes of entity framework for accessing data. In this article we will learn how to add System.Data.Entity namespace because Entity framework.dll missing by default. So first to add entityframework.dll file as a reference.
How to downloadStep-1 : Select Package Manager console from Tool menu.

Step-2 : WritePM> Install-package entityframework

Note : Internet connection must.
Now, Using this method you can download latest version of entityframework.